Warm Audio - TB12 Tone Beast ORANGE - No Signal

[p0p0p0000]

I have been probing like a madman, trying to trace the circuit for continuity, so far all the diodes have tested what i think is ok giving around 1Mohm resistance. And i haven't located a short in the caps or regs.. Still searching for that burnt resistor value cause that obviously needs to be replaced. I assume looking for a short involves looking for 0 resistance across a component or path where you expect some resistance??.. I'm not sure of the specifics of which components can and cannot be tested in circuit either, and don't know if this is even relevant when looking for a short.. Is it possible that the resistor was just at the end of its life and there is no short?? Can you determine if a device(with no missing components) has a short or not, by just measuring the power input jack somehow (for overall resistance or something)?? I have heard about dim bulb testers for repairing power amps etc with shorts, and am considering buying and or making one.

 

[Khron]

I have been probing like a madman, trying to trace the circuit for continuity, so far all the diodes have tested what i think is ok giving around 1Mohm resistance. And i haven't located a short in the caps or regs.. Still searching for that burnt resistor value cause that obviously needs to be replaced. I assume looking for a short involves looking for 0 resistance across a component or path where you expect some resistance??.. I'm not sure of the specifics of which components can and cannot be tested in circuit either, and don't know if this is even relevant when looking for a short.. Is it possible that the resistor was just at the end of its life and there is no short?? Can you determine if a device(with no missing components) has a short or not, by just measuring the power input jack somehow (for overall resistance or something)?? I have heard about dim bulb testers for repairing power amps etc with shorts, and am considering buying and or making one.

With only a single AC input voltage, it's likely the idea of the power supply is something like this:

https://sound-au.com/power-supplies.htm#s62

"Positive" voltage on the top, "negative" voltage on the bottom, and the mid-point is circuit ground.

The resistor that got burnt in your PSU, has a good chance of having a "twin" in R36. Especially if those four tall electrolytics between them are identical, those would seem to be that sort of "full wave voltage doubler" arrangement, albeit with an extra RC filter stage (R36 & C21, and R39 & C25 respectively) after the first pair of capacitors (C20 & C24).

And this would appear to be on the negative supply rail.

The phantom power side is likely comprised of a multi-stage voltage multiplier.

https://sound-au.com/articles/rectifiers.htm#s7

 

[p0p0p0000]

With only a single AC input voltage, it's likely the idea of the power supply is something like this:

https://sound-au.com/power-supplies.htm#s62

"Positive" voltage on the top, "negative" voltage on the bottom, and the mid-point is circuit ground.

The resistor that got burnt in your PSU, has a good chance of having a "twin" in R36. Especially if those four tall electrolytics between them are identical, those would seem to be that sort of "full wave voltage doubler" arrangement, albeit with an extra RC filter stage (R36 & C21, and R39 & C25 respectively) after the first pair of capacitors (C20 & C24).

And this would appear to be on the negative supply rail.

The phantom power side is likely comprised of a multi-stage voltage multiplier.

https://sound-au.com/articles/rectifiers.htm#s7

Thank you immensly, Ill read up and get back soon.

 

[p0p0p0000]

Thank you Khron, from my limited understanding your analysis seems spot on. I had an inclination the burnt resistor was a twin with r36 which seems to be in perfect condition. Would this suggest i should replace both as a set?.. I'm still unsure of its actual value, i think its 68ohms 1%tolerance but don't know the wattage, i guess i could just take it out and take it to the parts store or even just take the whole supply since its so small. The four tall caps are indeed identical 1000uf 50V and just as i was reading those values i noticed one (C25) has some greenish residue running down the side near where the resistor burnt, (perhaps a blown cap) or maybe from the resistor smoke??.. I'm not sure exactly how to proceed, considering pulling out caps and or diodes to test..

 

[Khron]

Well, what you could consider doing is:

- disconnect the power supply, and identify which of the four pins does what
- switch your meter to diode-check mode, and on the cable going to the audio board, apply red probe to the -15V (or whatever the regulator is set to), and black to "ground"
- if the meter is reading between 0.6-0.8V, then the downstream circuitry "should" be fine; if it's lower than 0.5V, then there's possibly a fault there somewhere

R36 you can measure in-circuit, you should get a sane-enough measurement. Eyeballing the size, that'll be at least 1W, if not even 2W, but the size-to-power-rating may well vary between manufacturers.

 

[p0p0p0000]

The power supply board is already disconnected and out of the case and I have removed the burnt resistor.
In diode mode ->| i get 0.638V when i have the red probe in pin 1 and black in pin 3 of the audio board cable as pictured (sounds like a win )
R36 measures 64.8 ohms in the circuit.. so i think my 68ohm figure is correct? Should i just replace both and is it worth going 3W? I am still concerned about the cap aforementioned too.

 

[Khron]

Should i just replace both and is it worth going 3W?

They don't cost a fortune, and if they physically fit, sure.

I am still concerned about the cap aforementioned too.

Considering they're some far-eastern never-heard-of brand, that can't hurt either. Try to source some Panasonic / Elna / Rubycon / Nichicon / United Chemi-Con replacements. 105C rated, general-purpose is fine (no point in springing for any fancy low-ESR for this application). Just double-check they're the suitable diameter (likely 5mm pitch, but they make 10mm and 13mm diameter ones)

 

[p0p0p0000]

I have ordered an ESR meter so I can measure capacitors. Still waiting on more dialogue with warm audio. I am still pondering what actually caused the resistor to fail, my prime suspects being the caps + diodes + regulators, new AC adapter, my house AC power and or cold solder joint(s).. I am trying to diagnose and understand the failure before attempting repair. I have noticed that the power section has changed on the MK2 black units in that there is no longer a DC jack, it now uses an IEC plug and also there is a ground connector on the chassis of the new units. I wonder if blown power supplies were a common issue for the units, possibly some kind of grounding issue causing problems?? I have also noticed that my new 24V AC adapter has 3 prongs, and is a class 4 transformer whilst the original US supply (warm sent me a photo) is a class 2 transformer.??.. Sooo Many things to consider... neurons firing ..

 

[callgrlmusic]

They don't cost a fortune, and if they physically fit, sure.



Considering they're some far-eastern never-heard-of brand, that can't hurt either. Try to source some Panasonic / Elna / Rubycon / Nichicon / United Chemi-Con replacements. 105C rated, general-purpose is fine (no point in springing for any fancy low-ESR for this application). Just double-check they're the suitable diameter (likely 5mm pitch, but they make 10mm and 13mm diameter ones)

accidentally ordered gigantic ones for this project bc i forgot to double-check specs.

 

[Khron]

I have ordered an ESR meter so I can measure capacitors.

Useful, but not so much in this case. Cheap capacitors are pretty much "replace on sight", if you want any sort of meaningful longevity / reliability for your gear.

Still waiting on more dialogue with warm audio.

Good luck with that, i wouldn't hold my breath. And there's no guarantee they didn't farm out the design of any of the stuff they're selling, so they may well have no tech support to offer in the first place.

I am still pondering what actually caused the resistor to fail, my prime suspects being the caps + diodes + regulators, new AC adapter, my house AC power and or cold solder joint(s).

I have seen failed electrolytics end up melting connectors between the transformer and rectifier bridge "upstream" of them.

 

[p0p0p0000]

I appreciate any/all feedback here. I have just noticed that the proper supplied AC adapter is 10VAC output whilst the one i have purchased is 24VAC output, not sure if this could be the cause but though it might be a factor. I am getting the vibe that Khron definitely suspects the caps are the cause, would you suspect the 4 large ones (or 1 of) to fail first? i am especially suspect of the closest cap and diode to the burnt resistor.. I am definitely going to use a dim light bulb to test for shorts.. something else i need to purchase. Could (should) i use solid or hybrid caps (where available for particular values) for higher reliability longevity and lower ESR (i.e an upgraded power supply)??

 

[Khron]

I have just noticed that the proper supplied AC adapter is 10VAC output whilst the one i have purchased is 24VAC output, not sure if this could be the cause but though it might be a factor.

Well, 2-4x the expected dissipation in a resistor WILL cook it in no time, so... that couldn't have helped things much Although am i misremembering what voltage the markings on the back of the unit are calling for (if any)?

I am getting the vibe that Khron definitely suspects the caps are the cause

Not impossible.

would you suspect the 4 large ones (or 1 of) to fail first?

Not in particular.

Could (should) i use solid or hybrid caps (where available for particular values) for higher reliability longevity and lower ESR (i.e an upgraded power supply)??

Not worth the expense, plus they need to be de-rated for operation at mains frequency (as opposed to 100kHz and above, where those parameters actually REALLY matter much more).

 

[p0p0p0000]

The marking on the back of my orange Mk1 unit (with DC jack)is 24V AC , the marking on back of MK2 unit (black with IEC plug) is 24 AC ONLY!! both are in pics within thread. This is one reason i suspect powering it with AC/DC adapter could be root cause... Anyhow, how exactly would you suggest i proceed?? Replace resistor(s)?? replace ALL?? caps, what about diodes and regs?? i could take out all the components (1 by 1?) and test, ESR meter should arrive in a few days...

 

[Khron]

Replace resistor(s)?? replace ALL?? caps,

Yes.

what about diodes and regs

That's test-able (also with the diode test mode on your meter), even in-circuit (especially after you remove those two resistors & four capacitors).

 

[p0p0p0000]

aight.. ill do that in a minute and report back... should i expect similar readings across each diode and across each reg (not really sure what to look for)?

 

[Khron]

should i expect similar readings across each diode

Yes, in the region of 0.6v forward voltage drop.

across each reg (not really sure what to look for)?

Positive regulator - red probe on output or ground, and black probe on input, should both give ~0.6v or more (up to OL / out-of-range).

Negative regulator - reverse probe colours.